By Polina Nikolskaya and Tom Balmforth DONETSK, Ukraine/MOSCOW (Reuters) -Russia’s strategic nuclear forces held exercises overseen by President Vladimir Putin on Saturday, and Washington accused Russian troops massed near Ukraine’s border of advancing and being “poised to strike”. With Western fears of war rising, the White House said U.S President Joe Biden’s national security team told him they still believed Russia could launch an attack in Ukraine “at any time” and he planned to convene his top advisers on Sunday to discuss the crisis. Foreign ministers from the G7 group of rich nations s…